Wireless communication technology including cellular phones has been widely used. Known cellular phones use a quasi-microwave band with a range of 800 MHz to 1.5 GHz. However, since the information content has been increased, wireless communication techniques using carrier frequencies in the millimeter wave band, which is higher than the microwave band, have been proposed and are in practical use. Wireless communication circuits for such high frequencies are expected to be used for mobile communications and network devices and increased in importance because such circuits are used Bluetooth and ITS (Intelligent Transport System).
In order to achieve the high-frequency circuits, materials for substrates included in the circuits must have superior high-frequency propagation characteristics at desired frequencies, that is, 1 to 100 GHz. In particular, ceramic substrates have attracted much attention because the following requirements are necessary to obtain such high-frequency propagation characteristics: low dielectric loss, high processing accuracy, and high dimensional stability.
However, known materials for ceramic substrates have high dimensional stability but low micro-processing accuracy; hence, satisfactory characteristics cannot be obtained at high frequencies. In order to improve the microprocessing accuracy, the following technique is disclosed in Japanese Unexamined Patent Application Publication No. 6-202323: a technique for forming via-holes in a green sheet containing a photosensitive ceramic composite by a photolithographic process. However, in the technique, there is a problem in that via-holes having a diameter of 100 μm or less cannot be uniformly formed in the sheet having a high aspect ratio, for example, a thickness of more than 50 μm in an accurate manner because the photosensitive ceramic composite has low sensitivity and resolution.
Since known photo-cured photosensitive green sheets have low elongation and tensile strength, there is a problem in that the sheets are damaged in sheet-handling steps such as a step of forming via-holes in the sheets peeled off from a film, a step of filling the via-holes with conductive paste, a step of forming a conductive pattern on the sheets, and a step of stacking the resulting sheets.
A process for preparing multilayer substrates using a ceramic material includes a step of forming via-holes in ceramic green sheets; a step of filling the via-holes with conductive paste or conductive metal powder; a step of forming a conductive pattern, for forming electrodes and/or circuits, on the resulting sheets; a step of stacking the sheets having the via-holes and conductive pattern to press the resulting sheets to form a green compact; a step of cutting the green compact into pieces, having a desired size, for preparing the substrate; and a step of firing the obtained pieces. In the firing step, the pieces are shrunk by 10% to 20%. Since the shrinkage is not necessarily uniform, the dimensional accuracy is lowered and the yield is therefore reduced.
On the other hand, substrates for circuits principally used for high-frequency wireless communication are used to manufacture mobile apparatuses as described above. Therefore, in order to increase the wiring density, the accuracy of processing via-holes must be enhanced and the diameter of the via-holes must be reduced. In addition, the shape and volume of modules including components are limited when the modules are placed in housings of the mobile apparatuses. Furthermore, since the directivity and sensitivity of internal antennas are greatly varied depending on the shape of a dielectric material, the degree of freedom in trimming must be high. However, since ceramic materials are hard and brittle, the trimming thereof seems to be difficult and has been hardly tried due to the low machinability.
The technique for processing the via-holes in the ceramic substrate by a photolithographic process has been disclosed as described above. However, in order to reduce the size of the mobile apparatuses, demands for microprocessing are increased; hence outer regions of packages must be machined.
The following apparatuses usually include glass ceramic multilayer substrates in principle for the above reasons: compact wireless terminals such as cellular phones and PDA (Personal Digital Assistance), image information systems such as digital video cameras and navigation systems for automobiles, and personal computers having a wireless communication function. In those apparatuses, there are many requirements for the package shape in view of the portability, the reduction in size, and the resistance to physical impact. This is because the following problems must be prevented since the portability and the reduction in size are demanded: a decrease in packaging volume available in the apparatuses, irregularities inside housing members, and interference between other components and modules.
A current technique for trimming sheets usually uses an NC punching machine or a die and has the problems below.
(1) A punching process using such an NC punching machine causes an increase in time to form continuous cutting lines.
(2) A punching process using such a die gives high productivity but causes an increase in cost because of the preparation of dies that can cope with arbitral shapes. The process also causes an increase in the number of manufacturing steps because a new die must be prepared when the substrate design is slightly modified.
(3) As is common with the NC punching machine and the die, the diameter of through-holes and via-holes is 0.1 mm or more because the minimum diameter of pins is about 0.1 mm. Therefore, the reproducibility of the shape is unsatisfactory in some cases when the external shape is machined.
(4) As is common with the NC punching machine and the die, green sheets formed on a sheet and then trimmed are isolated from each other when the external shapes have independent patterns. A step of stacking the resulting sheets is extremely complicated because the sheets must be aligned with each other, and the misalignment between the independent patterns is large.
As described above, the known machining processes are unsatisfactory for the sheet trimming. Therefore, the following substrate is proposed: a multilayer glass-ceramic substrate having an arbitrary external shape that is extremely fine and can be formed by a simple process, for example, a photolithographic process used for microprocessing via-holes or the like on a trial basis.
When sheets are trimmed by such a photolithographic process, the surface area of developed regions of the sheets is greatly different from that of other regions in contact with a developing solution in a developing step as compared with the processing of the via-holes. Therefore, the swelling rate of the sheets placed in the developing solution must be maintained constant and low. When the trimmed sheets are stacked and then fired, warpage and distortion can occur and cracks may be formed in some cases because stress concentration occurs in the sheets.
Since the trimmed sheets have independent patterns formed by trimming, the sheets must be processed in steps between the developing step and the stacking step in such a manner that the flexibility of the sheets is maintained.
In order to obtain satisfactory characteristics at high frequencies by increasing the degree of microprocessing source materials of ceramic substrates having high dimensional stability and a low dielectric loss tangent, fine via-holes having a high aspect ratio must be formed by a photolithographic process.
Furthermore, the sheets processed or not must be in a suitable condition fit for a step of forming a multilayer substrate in addition to the formation of such via-holes. Therefore, the photo-cured sheets must have an elongation, tensile modulus, and strength that are substantially equal to those of ordinary non-photosensitive green sheets.
A microprocessing technique using the photolithographic process is useful not only for processing the via-holes in the multilayer substrate but also for reducing the size of modules and for enhancing the performance. Therefore, a practical technique for processing the multilayer substrate by a photolithographic process is critical.
It is an object of the present invention to provide a ceramic material that can be microprocessed by the photolithographic process and are fit for a step of stacking the sheets to prepare the multilayer substrate as described above and also provide a new method for processing the multilayer substrate containing the material and a circuit substrate obtained by the method.